The present invention relates to an optical slub catcher which, inter alia, provides uniform lighting of the optical measurement head window as well as efficacious compensation for the soiling, the thermal drift, and the aging of such optical measurement head, and the disturbances created by ambient light. By normalizing signals and by using hybrid digital/analog processing, the present invention, allows, besides the traditional checks on the yarn, also laboratory-typical tests of yarn characteristics with a high degree of precision and stability such as determination of the coefficient of % variation in diameter (also indicated as "% VC", equal to the mean square deviation, as a percentage of the average value of the yarn diameter), the spectrogram or spectral analysis of the irregularities occurring in the yarn, as well as very minor periodical irregularities (Moire) occurring in the yarn. Consequently, the present invention is particularly suitable for application on open-end weaving machines.
From the prior art, various designs of optical slub catchers are known that are suitable for the detection of the yarn defects typical of the coners, such as slubs and/or attenuations of diameter, more or less extended lengthwise. The increasing demand by the weavers for better quality yarns has caused a continuous improvement in weaving techniques, which in the open-end process has resulted in defect levels much lower than those typical of the traditional weaving machines, thus creating a demand for more sensitive slub catchers. On the other side, in the open-end process, further defects, typical of the open-end weaving, occur, which must be detected and eliminated. Among these, the most typical and feared is the so-called Moire, which compromises severely the quality of the yarn, creating local slubs and attenuations in diameter, which occur at constant distance (they are periodical) and which, even if they are of small extent, can give rise to troublesome veins.
Therefore, to detect all of these defects typical of the open-end process, a slub catcher is required that is endowed with characteristics of resolution, precision and stability of the measurements considerably higher than the present standards, such enhanced characteristics basically resulting from a better uniforming of the light throughout the window of the optical detector, by means of an efficient compensation of the thermal disturbances, of the dirt and of the decay, as well as a compensation for the outer lighting. On the other side, the detection of Moire defects requires clearly also the provision of a suitable digital filter which, by allowing the signals to be processed in a digital way, may facilitate not only the detection of Moire, but also the detection of the spectrogram and, above all, the detection of the percent variation coefficient, also defined as "% VC" by the weavers, which characterizes in the most univocal way the real quality, or better described, the irregularity of the yarn.
In the slub catchers of the prior art, various means and contrivances have been used for the purpose of uniforming the measurement light beam throughout the window, such as the bending of the emission and reception plane of the window, the reflection of the light beam on a homogenizing mirror, as well as the correction of the reflection of the mirror by providing lines and reliefs thereon, and the insertion of diffuser filters or grids. But all of these contrivances, however, lead to a decrease in the energy of the light beam, worsening the signal-disturbance ratio, thus giving results which, if can be enough for the slub catchers for coners, are not so for the precision required by the open-end process, above all for the determination of Moire and of the spectrogram.
A second serious drawback of the slub catchers of the prior art is the change in photodiodes emission with temperature.
Also, the photoreceivers suffer from the same type of defect, so that by a proper selection of the components, obtaining a certain degree of compensation is possible, but on the condition that both the elements are at the same temperature.
In the geometrical arrangements of the prior art, however, the receiver and the emitter are always installed on opposite sides, so that, because of their distance during the heating transients following the turning on, or because of an outer irradiation, the two elements can have different temperatures, consequently causing considerable errors.
The extent of such errors can be acceptable for the degree of precision required by the coners, but not at all for the open-end process in which the measurement of Moire requires a higher degree of precision.
As for decreasing or elimination of the outer disturbances caused by the ambient lighting, the prior art has not solved the two problems of the saturation of the amplifiers and the presence of variations in the signal constituting the measure of the diameter, always in the presence of the disturbance created by outer lighting. Both problems result because of the incorrect use of the diode and because of a different dynamic answer of the amplifier circuits when the signals reach very different levels.
According to the present state of the art, no means exist for reducing the phenomenon of saturation but for the installation of filters positioned in front of the receiver, which attenuate all the wave lenghts different from the I.R. band inside which the emitter works.
On the market, no optical slub catcher exists, which is capable of withstanding a light slash due to direct or reflected sun light entering the weaving room.
The collected and amplified signal is always the total signal, so that a gain in the first amplifier stage, optimum for the modulated signal, amplifies obviously also the light, which may reach very high intensity levels. As for the apparent change in yarn diameter in the presence of disturbing light, inasmuch as the disturbing light is of small intensity, the equipment of the prior art has not corrected this problem because it is not critical to the precision typical of the coners.
The equipment of the prior art, completely analog as it relates to the solution of the problem of the compensation for the dirt and of the optical efficiency, both from the viewpoint of the decay and of the thermal disturbances, is provided at most with a type of circuit, wherein all the types of degradation which cause the output to slowly change, are compensated for by just varying the emission so as to keep constant the average output value corresponding to the preset average value of the yarn diameter.
In a typical arrangement, an integrator, with very long time constant, integrates the difference between the signal and the desired average value, and its output is delivered as the emission level set.
The limit of this approach is that the emission is also always varying in the presence of diameter changes of more or less long periodicity and that the compromise involved in executing the calibration of the integration time constant so that a sufficient dynamics of compensation for the dirt be achieved, cannot prevent at the same time what the system may fit in, e.g., a double yarn which is inserted as slowly-varying diameter.
In any case, the detection of the Moire-effect causing defects is only achieved by the prior art when the peaks due to the thickening are so large as to be computated as a chain of knops, i.e., as a not necessarily periodical chain of small slubs.
This approach leads to the possibility of stopping the occurrence of Moire defects only when these are already well evident because detecting the periodicity of the attenuations and of the slubs by using simple systems is not possible.
In the solutions of the prior art, therefore, for the purpose of stopping the occurrence of Moire defects, it is necessary either to impose a very narrow limit to the irregularity of the yarn or to allow a certain irregularity, which can allow already well-evident Moire levels to pass.
Keeping well separate the two measurements is, on the contrary, a weavers' requirement, in that the case could occur when a relatively considerable irregularity is acceptable without renouncing the stopping of just visible Moire levels.
As it has already been anticipated for the Moire, and for the other type of defect, the irregularity of the yarn, the analog means of the prior art carry out a measurement which is only approximately correlated to the most precise definition of the irregularities which is the VC, variation coefficient, mathematically defined by the mean square deviation as a percentage of the average value of diameter.
In the solutions of the prior art, the irregularity is not measured and then possibly compared to a limit, but rather the peaks, i.e., the number of times the measured diameter exceeds a prefixed value, are counted and the stop for excess of irregularity is given when the number of these events exceeds a prefixed value.
This calibration by the prior art is consequently the same for the Moire defect. But even if it allows the occurrence of determined levels of irregularity to be stopped, these levels do not coincide with the % VC level; moreover, when the stop does not occur, a measurement of the actual value of % VC is not available for the accepted yarn.
The spectrogram, or spectral analysis of the irregularities existing in the yarn, is a typical function presently performed by dedicated laboratory equipment, which have such a cost and complexity as to render the spectrogram applicable to yarn samples, but relatively not much representative of the actual quality of the whole production.
The computing of the spectrogram directly on the slub catcher has never been done because the prior art is analog.
Nor does the availability of a microprocessor on each slub catcher automatically solves this problem because the velocity and complexity of the operations, besides the storages required by the computation of Fourier coefficients, are not compatible with the structure and the cost of the microchips available on the market as well as useable for the slub catching control.